The change was about skipping the direct copyRecord() call for
records that will be processed inside another copyRecord() call
for the record of the default language

Due to https://review.typo3.org/#/c/51070/ $overrideValues contains

['l10n_source' => 0]

When an element is copied inside an earlier copyRecord() call
the error log message is generated which leads to an error
flash message editors see when copying a page, but everything
(copying the records once) went fine.

A earlier call of copyRecord() can occur when using extension like
gridelements when copying a content element laying inside a
grid container element.

The following public properties have been removed
* TYPO3\CMS\Backend\FrontendBackendUserAuthentication->extAdmEnabled
* TYPO3\CMS\Backend\FrontendBackendUserAuthentication->adminPanel
* TYPO3\CMS\Backend\FrontendBackendUserAuthentication->frontendEdit
* TYPO3\CMS\Backend\FrontendBackendUserAuthentication->extAdminConfig

The following PageTSconfig option has no effect anymore:
* TSFE.frontendEditingController

These constant redefintion guards were forgotten to be updated when
Bootstrap::getInstance() and the call to SystemEnvironmentBuilder::run()
in Bootstrap::baseSetup() were removed.

These guards are no longer necessary for regular execution as baseSetup()
does no longer (conditionally) invoke SystemEnvironmentBuilder::run()
(which was provided for deprecated and now removed bootstrapping code).

We do still leave the guards in, as they are needed in unit tests,
but update the comments.

Remove code that is not needed due to the static bootstrap
and PSR-15 refactoring during v9 phase.

All deprecated methods and state is removed from the Bootstrap class,
which includes the Bootstrap instance itself. Bootstrap is provided as
statically invoked utility now.
The Bootstrap instance was deprecated implictly due to the deprecation
of getInstance() (combined with the protected constructor).
We therefore remove the constructor and all (now unused) class properties
and the method-chaining (which was only useful to reduce calls to the
getInstance() method).

$GLOBALS['TYPO3_GLOBALS_EXT'] was deprecated in
https://review.typo3.org/58408 and is now removed together
with the internal helper classes LoadedExtensionsArray
and LoadedExtensionArrayElement which powered this array.

Core v10 compatible releases of ext:introduction and
ext:bootstrap_package will take time until the other day.
To not disable all install related tests, the
ext:introduction related ac tests are skipped for master
for the time being.

Due to recent security releases and fixes against cross-site scripting
in central modal window component, templates are separated in order to
distinguish between (secure) HTML and attribute or text nodes.

Using large media files (*.youtube, *.vimeo in the TYPO3 core)
might lead to denial of service scenarios. In order to avoid
that, media files are limited to have a content size of 2048
bytes as a maximum. Usually these files contain just the remote
identifier - thus, ~20 bytes should have been sufficient already.